(CNN) -- It may not be legal, but it's definitely popular. For just 10 Guatemalan quetzals, or 15 Mexican pesos (the equivalent of just over a U.S. dollar), you can pay to ride a zip line across a river and into a new country.

A news team from Mexico's Televisa network, a CNN affiliate, found four zip lines crossing over the Suchiate River, which serves as part of Mexico's southeastern border with Guatemala. They observed people crossing into Mexico in broad daylight, apparently not worried about immigration authorities posted not far from there.

Guatemalans on both sides of the border are in charge of the zip line and collecting money from people willing to cross.

A Guatemalan immigrant getting ready to cross the border using the zip line admitted he was doing so illegally. He said he had "obtained my passport but didn't know I also need a visa to cross" into Mexico.

Mexico shares a largely unpopulated, almost 600-mile border with Guatemala marked by jungle and rugged mountains. The zip lines connect the Guatemalan community of El Carmen to the Mexican town of Talisman.

Once in Mexico, migrants travel on freight trains or by bus to the U.S. border, although many never get there. Last week, Mexican authorities stopped a bus in the central Mexican state of Queretaro with 104 undocumented migrants on board. Last August, 72 migrants were killed by a Mexican drug cartel in a ranch in San Fernando, Tamaulipas, only 100 miles south of the U.S. border.

Those who do get to the U.S. border have another river to cross, the Rio Grande (or Rio Bravo, as it's known in Mexico) or a 21-foot wall.

Many are now going underground.

On a recent trip to Arizona, U.S. border patrol agents gave a CNN crew access to a tunnel being illegally used for this purpose. The mile-long tunnel was built to prevent flooding in the border city of Nogales, Arizona, but it has been increasingly used in recent years to smuggle immigrants and drugs.

U.S. Border Patrol Agent Ariel Medeles says crossing this way can be dangerous. "There was a group down here and then they got caught in the flash flood and hours later they were looking for a body north of the openings," says Medeles.

Some immigrants die trekking across the treacherous Arizona desert, but many thousands more make it to the land of their dreams, after a trip of hundreds -- even thousands -- of miles.